1. Field of the Invention
The present invention relates to a vehicle motion control apparatus, particularly relates to the vehicle motion control apparatus for regulating a hydraulic braking pressure supplied to at least one wheel brake cylinder out of a pair of wheel brake cylinders included in a hydraulic circuit, to restrain an excessive oversteer and/or an excessive understeer, thereby to maintain stability of a vehicle in motion.
2. Description of the Related Arts
As for a vehicle motion control apparatus, there is disclosed in Japanese Patent No. 3058172, which corresponds to the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling vehicle motion, by determining a desired yaw rate of a vehicle, and controlling braking force in response to a comparison of the desired yaw rate with a sensed actual yaw rate of the vehicle to maintain a vehicle stability during the vehicle motion. Also, in Japanese Patent Laid-open publication No. 10-129441, which corresponds to the U.S. Pat. No. 5,913,578, proposed is a vehicle motion control system for setting desired slip rates for use in oversteer restraining control and understeer restraining control on the basis of a vehicle state, and regulating hydraulic braking pressure in each wheel brake cylinder on the basis of each desired slip rate.
In Japanese Patent No. 2884789, as front wheels tend to be locked earlier than rear wheels according to an ordinary vehicle, it has been noted as an issue to be solved that the vehicle tends to be in the understeer state, when a rapid braking is made while the vehicle is turning along a curve, with high lateral acceleration applied to the vehicle. In order to prevent that state, proposed is an anti-skid control apparatus for decreasing the braking force applied to wheels located on the outside of the curve to be smaller than the braking force applied to wheels located on the inside of the curve, thereby to apply a yaw moment for canceling the tendency of the understeer state. And, it is proposed to set a slip rate for use in an anti-skid actuator provided for the wheel located on the outside of the curve to be smaller than the slip rate provided for the wheel located on the inside of the curve, with the lateral acceleration being made higher.
According to the system and apparatus as disclosed in the Japanese Patent No. 3058172 and publication No. 10-129441, a control for maintaining a stability of the vehicle in motion (i.e., vehicle stability control) is performed. Therefore, an excessive understeer can be restrained by regulating the hydraulic braking pressure in a wheel brake cylinder out of a pair of wheel brake cylinders included in a common hydraulic circuit, for example. In general, in order to restrain the excessive understeer, the hydraulic braking pressure is supplied to the wheel brake cylinder operatively associated with the wheel located on the inside of the curve. In the case where the wheel located on the inside of the curve has been apart from a road surface to be positioned in a floating state, however, supplying the hydraulic braking pressure to the wheel brake cylinder operatively associated with the wheel located on the inside of the curve could not act as the understeer restraining control. In this case, therefore, it is preferable to supply appropriate hydraulic braking pressure to the wheel brake cylinder operatively associated with the wheel located on the outside of the curve, thereby to increase the vehicle deceleration, to result in restraining the excessive understeer.
In the mean time, according to the system and apparatus as disclosed in the Japanese Patent No. 3058172 and publication No. 10-129441, the control is not necessarily undertaken when a brake pedal is depressed, so that the operation of the brake pedal is not used for a condition for undertaking the control. As the Japanese Patent No. 2884789 relates to the anti-skid control apparatus, which may be included in the category of the system and apparatus as described above, means for solving a specific issue for the anti-skid control may not be used to solve various issues caused in the vehicle motion control performed by the system and apparatus as described above. For example, it has been employed as the issue to be solved, in Japanese Patent No. 2884789 as described above, that the vehicle tends to be in the understeer state, when the rapid braking is made during the vehicle's turning operation, and it has been proposed as a countermeasure that the slip rate provided for the wheel located on the outside of the curve is set to be smaller than the slip rate provided for the wheel located on the inside of the curve. That countermeasure does not directly relate to applying the braking force to the wheel located on the inside of the curve, for example, when the understeer restraining control is undertaken during the vehicle's turning operation, according to the system and apparatus as disclosed in the Japanese Patent No. 3058172 and publication No. 10-129441, and it is not applicable for solving the issue caused in the state where the wheel is positioned in its floating state during the understeer restraining control, as described before.